Article furnished courtesy of RMC Pacific Materials
By George Allen, Plant Manager
Mount Diablo Review, Spring 2004
Two partners, Harrison and Birdwell, developed mining for construction aggregates on Mount Zion in 1947. However, this was not the first mining activity to occur in the area. The discovery of coal in approximately the 1850's brought about the first commercial mining and the subsequent settlement of coal mining towns such as Stewartville, Somersville, and Nortonville.
The discovery of coal also led to additional prospecting wherein mercury and copper were discovered. It has been written that a "copper rush" occurred in the 1860's around Mount Diablo.
Copper was never found in significant quantity to make commercial mining worthwhile but the Mt. Diablo mine did produce mercury until the 1950's. Gold and silver were also reported in various assays but, again, none was ever found in significant quantity to make commercial mining worthwhile.
As a result of the search for the precious metals, mines and/or claims with names such as Great Republic, Superior, Pioneer, Horse Shoe, Rising Sun, San Carlos, Rough and Ready, and numerous others were developed. Today, the diggings of the Mount Zion Copper Company, Summit of Zion, San Pedro, and a few other mines can still be seen.
In 1947, Harrison and Birdwell developed the Clayton quarry. The first excavation was behind the current shop building, but it was shortly moved to the east and south because of better rock on that side. In 1954, Harrison and Birdwell sold out to Pacific Coast Aggregates (later named Pacific Cement and Aggregates), who in turn sold out to Lonestar Industries in 1965.
In 1988, Lonestar partnered with RMC of London, England. Today the quarry is owned by RMC Pacific Materials, a wholly owned company of RMC Ltd. of England. The quarry is located on Mount Zion and the material mined is diabase, a hard, durable rock used in the construction of roads, buildings, rail lines, dams, and levees. Over 25 different products are made to supply construction needs.
The State of California has designated Mount Zion as a mineral reserve to be used to supply the construction needs of Contra Costa County. Of all the rock shipped out of the quarry, typically 72% of it goes to public sector projects. Some of the specific projects we have been involved in are B.A.R.T. (50% of all the rail ballast came from Clayton), Interstate 580 & Interstate 680 freeways, Highway 4, Highway 24, the new Giants Stadium, the Los Vaqueros Dam, Concord Naval Weapons Station, Delta levees, and many other projects.
The Clayton Quarry has been in continuous production for over 50 years, and expects to be producing for many years to come.
1. Mount Zion was also known as "Pyramid Hill".
2. Elevation of Mount Zion is 1635 Feet.
3. Minerals found in the quarry are malachite, azurite, iron pyrite, magnetite and calcite.